1. Field of the Invention
The present invention related to the selective detection of metal ions in solution or other analyte medium, and more particularly to a fiber optic-based real time, continuous process and apparatus for the highly selective detection of metal ions at nanomolar concentrations.
2. Description of the Related Art
Fiber optic chemical sensors are devices of growing importance in fields as diverse as oceanography, chemical process control, in vivo clinical diagnosis, and environmental monitoring. Fiber optic sensors have the capability of continuously monitoring the level of an analyte in a sample that may be up to kilometers away from the instrument, or in an inaccessible space. A large fraction of these sensors exploit a change in photoluminescence (e.g., fluorescence) in response to the analyte as a means to transduce its presence or level. Fluorescence-based sensing, and more generally photoluminescence-based sensing, is desirable due to its high sensitivity, good selectivity, and relatively straightforward instrumentation.
A very important potential application of fiber optic chemical sensors is in the field of chemical oceanography. One of the current issues in chemical oceanography is the difficulty of determining certain chemical constituents of seawater, such as trace metals, at many points in the ocean. The basis of the problem is the slow rate at which discrete samples can be obtained from deep in the water column and analyzed, usually by preconcentration followed by atomic absorption or emission spectrophotometry, or electrochemical methods. Such techniques permit analysis of only a few dozen samples per day because they cannot usually be stored, the analyses require skilled labor, and moreover the sophisticated instrumentation necessary is ill-suited to shipboard operation. The rate and ease of measurement are important issues in designing data collection efforts to understand phenomena occurring on a large scale, such as global climate change, especially given the high cost ($10,000 per day) and limited availability of ships where such analytical chemistry may be performed.
For example, it is of particular interest to monitor zinc (II) concentrations in seawater. Zn(II) is one of a group of metal ions (along with Fe and Mn) having particular importance because they serve as nutrients, and indeed are required as enzyme cofactors by many taxa of organisms for survival. These ions typically exhibit a concentration dependence with depth characteristic of nutrients (low levels in the photic zone where most plankton are, then increasing to a constant level at greater depths).
The difficulties of quantitatively analyzing Zn(II) in seawater exemplify many of the practical problems encountered with remote, real time quantitative analysis of environmental metal ions. Much Zn(II) is dissolved in the form of complexes with ill-defined organic ligands, such that free concentrations of these ions are typically in the subnanomolar range. Indeed, there are large regions of the ocean where low concentrations of one or more of these ions are believed to limit primary productivity of microorganisms. Zinc(II) levels in sea water have only recently been measured with accuracy, because of the great difficulty of avoiding contamination of the samples during collection and subsequent analysis. Stripping voltammetry has been the method of choice, with some effort being devoted to understanding the speciation of zinc ion in the ocean.
Other metal ions of interest as analytes include cobalt, copper, lead, mercury, cadmium, and nickel.
For trace metal analyses in sea water, the classical fluorimetric indicators such as morin or hydroxyquinoline sulfonate that have been incorporated into fiber optic sensors are ill-suited mainly because they are insufficiently selective. Even the excellent chelatometric indicators for calcium do not discriminate against similar concentrations of Zn or Mn when these are present, nor Mg at hundred-fold higher concentrations. By comparison, an indicator for Zn in sea water must exhibit million-fold (60 dB) selectivity with respect to magnesium, as well as other cations. Such selectivity is not to be found in typical synthetic chelators such as these, or ionophores. However, some metalloenzymes exhibit exceptional binding selectivity for the metal ions which participate as cofactors in catalysis. An example of this is the carbonic anhydrase from mammalian erythrocytes (carbonate hydro-lyase, E.C. 4.2.1.1), which functions in vivo by dehydrating bicarbonate to give CO.sub.2 (and vice versa), only the latter of which can pass through cell membranes.
A continuously monitoring in situ sensor would be desirable for these studies since it could be maintained at a desired depth and continuously report analyte levels as the ship proceeded along its course. Fiber optic chemical sensors have precisely these qualities, and recognizing these advantages Wait, Lieberman, and others have described fiber optic chemical sensors for use at sea.